[courier-users] Courier pythonfilter and whitelisting

[Flavio Stanchina]

First, apologies for sending a draft of this message that I had just
started editing. Please ignore that.

Now for the real stuff: I recently installed pythonfilter for its
greylisting abilities, and it's doing its job extremely well on a couple of
servers. Many thanks to those who invented greylisting and those who
implemented pythonfilter! ;)

I have some thoughts and questions for you. First, about auto_whitelist.
Initially I enabled it, but I noticed a rather serious drawback: these
days, it's common to receive spam with the sender's address set to yourself
or some other username in your domain, so if you email yourself (which I
sometimes do) or a colleague and have auto_whitelist enabled, you'll also
open the door to some spam. Is there any way to avoid that?

Then, about virus filtering. I think that using clamav before greylisting,
as suggested in the default configuration, is not the best course of action
because greylisting will hopefully keep most viruses out without having to
look at the message contents.

However, pythonfilter's architecture is such that if a filter accepts a
message, then all the following filters will be skipped. I'm of course
using various whitelisting modules before greylisting, so whitelisted
messages will skip clamav altogether if I place it after greylisting. While
I do not expect to receive viruses from most of the addresses I have
whitelisted, being paranoid doesn't hurt.

Therefore, I'd like to have whitelists apply only to greylisting and clamav
apply to everything that has not been rejected alright by greylist (or
whatever other filter I might place before clamav). In pseudocode, that
would be something like:

if (!whitelist_relayclients && !whitelist_auth && !whitelist_block)
greylist;

clamav;

Can this be done with the current pythonfilter? As a workaround, could I
run two pythonfilter instances, one for the greylisting and the other
running the clamav filter?

I'd be happy to write some code to implement, uh, let's call it "nonlinear"
filtering, but I would definitely want Gordon's and others' opinion on how
to do it properly.

--
Flavio Stanchina
Informatica e Servizi
Trento - Italy

Those who do not understand Unix are condemned to reinvent it, poorly.
-- Henry Spencer

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